Method and system for handling optical signals

ABSTRACT

A method and a system for performing pre-emphasis of one or more optical channels respectively carrying one or more optical signals, by effecting bit rate of the one or more of the signals, thereby adjusting BER value thereof.

FIELD OF THE INVENTION

[0001] The present invention relates to a method and a system forpre-emphasis in optical systems, such as multi bit rate WDM systems.

BACKGROUND OF THE INVENTION

[0002] In optical networks, chains of amplifiers and other networkelements usually add spontaneous noise, thus degrading the opticalsignal which causes appearance of transmission errors. One of the mainparameters of optical transmission—the required Optical Signal-to-NoiseRatio (OSNR) of a specific signal—depends on the initial signal power.The distance and number of amplifiers in the chain via which an opticalsignal is transmitted, also influence the overall OSNR and thus degradesthe overall network performance. Standards in the field of opticalcommunications (for example, ITU-T Standard Recommendation G.692, 10/98)refer to so-called pre-equalization for equalizing powers of opticalsignals which are degraded due to optical amplifiers' gain tilt. Thepre-equalization partially compensates amplifier gain variation and gaintilt using the following scheme. The highest channel power in the systemis assigned to the channel which will undergo the least line amplifiergain, whereas the lowest channel power is assigned to the channel thatwill undergo the most channel line amplifier gain. If pre-equalizationis not used, the amount of channel power difference at the transmitinterface leads to a reduction in the amount of amplifier gain variationand gain tilt which can be tolerated by the system.

[0003] U.S. Pat. No. 5,790,289 describes a wavelength divisionmultiplexed (WDM) optical communication method and apparatus using apre-emphasis technique to adjust the attenuation of a particular opticalchannel at a transmitter terminal to produce identical signal-to-noiseratios for all of the optical channels at a receiver terminal. Thepre-emphasis adjustments to the transmitted signals are made on thebasis of signal-to-noise ratio measurements performed at the receiverterminal. The signal-to noise ratio values for each channel aretransmitted through a facing line that is also used to transmit dataalong optical communication lines from the receiver terminal back to thetransmitter terminal.

[0004] U.S. Pat. No. 6,040,933 describes a method and apparatus forchannel performance equalization in wavelength division multiplexed(WDM) systems. Performance of the channels is estimated from opticalpower measurements of each signal transmitted by the channels. Themeasurements are taken at the inputs of optical amplifiers in thetransmission path of the system. The channels are equalized by adjustingthe optical power of the channel transmitters. The method can compensatefor signals having different bit rates by applying an offset to theamount of optical power adjustment of the channel transmitters.Furthermore, if different types of optical amplifiers are used in thetransmission path, the method can accommodate different noisecharacteristics of the amplifiers by using their noise figures indetermining the amount of optical power adjustment of the transmittersthat is required to equalize channel performance.

OBJECT OF THE INVENTION

[0005] The object of the invention is to provide a new method and asuitable system for effecting pre-emphasis in one or more channels of anoptical telecommunications system.

SUMMARY OF THE INVENTION

[0006] It should be clarified that the final purpose of performingpre-emphasis in optical systems, particularly in WDM systems withmultiple optical channels, is to obtain transmission with favorable(i.e., minimal) bit error rates (BER) and predetermined respectivequality of service (QoS) parameters. Those skilled in the art know thatthe Bit Error Rate (BER) of a particular optical channel is a functionof a plurality of factors, such as:

[0007] BER=f(BR, OSNR, P, FEC, CD, PMD, NL, etc.), where

[0008] 1. BR—bit rate of transmission via the particular opticalchannel,

[0009] 2. OSNR—optical signal to noise ratio measured at the end of theparticular optical channel,

[0010] 3. P—power launched in the particular optical channel,

[0011] 4. FEC—forward error correction feature which may be provided tothe particular channel,

[0012] 5. CD—chromatic dispersion effects active in the channel,

[0013] 6. PAM—polarization mode dispersion effects,

[0014] 7. NL—existence of non-linear effects in the channel,

[0015] and where factors 2, 5, 6, 7, in turn, are functions of distance.It is understood that in the optical Add/Drop architecture, each channelmay pass different distances.

[0016] The Inventors have come to the conclusion that the above objectcan be reached by a new method comprising adjusting parameters of thetransmitted signal(s) which have never been adjusted before, for exampleby affecting bit-rate of the signal(s).

[0017] To the best of the Applicant's knowledge, none of the presentlyknown optical single-channel or multi-channel systems utilizes thechannel(s) pre-emphasis (or adaptation, or equalization) by means of bitrate adjustment. It has been found by the Inventors, that in most casessuch an adjustment enables both main and fine tuning of the opticalsignal to finally obtain more accurate transmission having a betterQuality of Service.

[0018] According to one basic version of the method of adjusting BERvalue in one or more optical channels performing data transmission withrespective bit rates, the method comprises steps of

[0019] a) obtaining BER value for the one or more optical channels,

[0020] b) reducing bit rate in one or more of the optical channelshaving the obtained BER value exceeding a maximally accepted absolute orrelative BER value, thereby adjusting the BER value thereof.

[0021] According to one possible version of the method, it comprises apreliminary step of defining the maximally accepted absolute BER valuefor said at least one optical channel.

[0022] In another version of the method, i.e., in the case of amulti-channel is optical system such as a WDM optical system, the methodcomprises:

[0023] stating Quality of Service priorities for its optical channels asa set of priority ratios,

[0024] defining required ratios of BER values of the optical channels asa logically inverse set of said set of priority ratios, and

[0025] regulating the obtained BER values of the optical channels underconstraint of said priorities by adjusting bit rates of at least one ofthe optical channels to obtain real ratios of BER values of the opticalchannels substantially equal to said required ratios.

[0026] In the latter version, i.e., when adjusting BERs with taking intoaccount the prioritizing of the optical channels, there are achievednon-absolute, but optimal relative BER values of the optical channels.The prioritizing, i.e. the step of defining the mentioned set ofpriority ratios can be performed in a number of ways, for example:

[0027] based on the prices stated for transmission over differentoptical channels, i.e., the greater the price the higher the priority;

[0028] based on at least one required Quality of Service (QoS)parameter, for example, based on an acceptable delay time for aparticular type of service: the highest priority can be assigned fordata, the lowest priority—for voice; another example—based on the typeof service and an acceptable bit error rate (BER).

[0029] More particularly, the method of pre-emphasis of a multi-channelsystem can be performed by the following steps:

[0030] determining a set of priority ratios by finding, for a group ofthe optical channels, relations of stated priorities (say, by formingrelation(s) of predetermined prices, or relations of priority valuesconnected with particular types of service),

[0031] considering said set of priority ratios to logically present aninverse set of ratios between the required BERs of said group of thechannels, thereby obtaining a set of required BER ratios,

[0032] obtaining current BER values at each optical channel of saidgroup and forming a set of real BER ratios for the same group of theoptical channels,

[0033] comparing the formed set of real BER ratios and said set ofrequired BER ratios, and if not substantially equal for at least one ofsaid channels, performing the adjustment in said at least one opticalchannel,

[0034] obtaining BER values for at least the channel(s) where theadjustment were performed and forming a set of new BER ratios for saidgroup,

[0035] comparing the set of required BER ratios with the set of new BERratios,

[0036] continuing the adjustment until the set of new BER ratios issubstantially equal to the set of required BER ratios.

[0037] The step of determining a set of priority ratios may compriseconstructing said ratios based on a priority of one of said opticalchannels selected as a reference to the remaining ones from the group.In the simplest case, the group may comprise all the channels of thesystem.

[0038] The term “obtaining BER values” comprises either directmeasurement of BERs at each of the channels, or obtaining the BER valuesby calculations based on other parameters measured or given directly.For example, BER (Bit Error Rate or Probability of error) can beestimated, if Q (Quality factor of the signal) is known. Q can bedirectly measured, and BER can then be mathematically or graphicallyestimated based on the function

BER=½*eifc(Q/? 2)  (1)

[0039] which is known to those skilled in the art (for example, GovindP. Agrawal. Fiber-Optic Communication Systems. 2^(nd) ed. Wiley seriesin microwave and optical engineering. A Wiley Inter-science publication.1997, p.172-173).

[0040] In one particular and practical version of the method ofregulating BER value in one or more optical channels, each comprising atransmitter terminal, a receiver terminal, one or more amplifiers anddispersive fiber elements there-between, the method comprises additionaladjusting, in said at least one channel, one or more physical parametersfrom a non-exhaustive list comprising power (P), and characteristics ofdispersion.

[0041] The power and dispersion may be adjusted at any point of thechannel, while the bit rate—only at the transmitter.

[0042] The term “characteristics of dispersion” means at least onefactor from the list comprising CD and PMD.

[0043] The version of the method, where additional physical parametersare adjustable, is applicable both to approaching an absolute, and anoptimal relative value of BER in the optical communication channels of amulti-channel optical system.

[0044] It should be noted that the step of additional adjusting can beperformed at an arbitrary point between a transmitting terminal and acorresponding receiving terminal of a suitable optical channel. Sinceoptical channels may have different lengths, the step of additionaladjusting may comprise power de-emphasizing of optical signals passingshorter distances.

[0045] It has been realized by the Inventors, that each of theabove-mentioned physical parameters affects BER in a channel in itsspecific way. For example, increasing bit-rate in a channel willincrease the electrical bandwidth and hence—increase of the total noisereceived. Power increase in a particular channel, relatively to otherchannels in the system, leads to increasing the physical parameter OSNR(optical signal to noise ratio) which is always good for reducing BER,while simultaneously causes non-linear effects which lead to distortionsand hence to increasing BER in the particular channel. Likewise,adjusting the dispersion so that the total dispersion at the receiver isclose to 0 always improves BER. In other words, there is usually a tradeoff situation with regulation of the optical channels, such as a tradeoff between non-linear effects and amount of dispersion in a link at anygiven point, or a trade off between a signal power and a non-linearsignal distortion.

[0046] The above-described method can be explained based on thefollowing equation known for modeling optical systems [FabrizioForghieri, R. W. Tkach, D. L. Favin. Simple model of optical amplifierchains to evaluate penalties in WDM systems. IEEE Journal of lightwavetechnology, vol. 16. No.9, September 1998]. It should be noted that thismodel has not been ever used for controlling BER in multi-channeloptical communication systems, such as $\begin{matrix}{Q = {\frac{OSNR}{\sqrt{{2*{OSNR}} + 1} + 1}\sqrt{\frac{Bo}{Be}}}} & (2)\end{matrix}$

[0047] where

[0048] Q is a quality factor of the signal;

[0049] Bo—is the bandwidth of the optical filter at the receiver,

[0050] Be—is the bandwidth of the electrical filter of the receiver;

[0051] The electrical bandwidth is directly proportional to the bit rateutilized in the channel.

[0052] Therefore, if one increases Be (proportional to the bit rate),the required OSNR for preserving the constant Q must increase, as can beseen from the above equation. It means, that if one reduces the bit rate(and, proportionally, the Be), the required OSNR for preserving theconstant Q may be reduced. The equation (2) can also be used forestimating BER when Q is measured directly.

[0053] According to another aspect of the invention, a system is alsoprovided for performing pre-emphasis in at least one optical channel fortransmitting data, the system being capable of obtaining BER value at aterminal point of said at least one optical channel, processing saidobtained BER value and adjusting bit-rate at a transmission point ofsaid optical channel thereby regulating the BER value.

[0054] Further aspects and details of the invention will become apparentas the description proceeds.

BRIEF DESCRIPTION OF THE DRAWINGS

[0055] The invention will further be explained with reference to thefollowing non-limiting drawings in which:

[0056]FIG. 1a is an exemplary graphical diagram of dependency of thequality factor Q of an optical signal from the bit rate in the opticalchannel.

[0057]FIG. 1b is an exemplary schematic graphical diagram showing howBER=½*erfc(Q/? 2) depends on bit rate of an optical signal, taken forthe same range of bit rates as in FIG. 1a.

[0058]FIG. 1c is a schematic and simplified graphical diagram ofrelation between the BER and OSNR ratio in one or more optical channels.

[0059]FIG. 2 is a schematic block diagram of a system for performingpre-emphasis in an optical system, according to one version of theinventive method.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0060]FIG. 1 illustrates a simplified version of the graphical relationbetween a bit rate (BR) in a single optical channel and the (Q) factorof an optical signal transmitted through the channel. The graph is builtfor an exemplary range of bit rates, using equation (2) with consideringall other arguments in the equation to form a constant. The schematicgraph shows that generally, increase of the bit rate should lead toreduction of the Q factor in the optical channel, while degree of thereduction depends on specific ranges of the bit rate.

[0061]FIG. 1b. The drawing illustrates how the Bit Error Rate (BER)parameter of an optical signal can be influenced by changes of thesignal bit rate (BR) in a particular range of 2 Gbps to 6 Gbps. It canbe seen that in this range, changes of the bit rate bring fluctuationsof the BER. The graph is built for particular given values of OSNR,dispersion characteristics and system parameters. It should be notedthat if OSNR value decreases, the graph will shift leftwards, i.e. toachieve a desired BER value at a decreased OSNR a lower bit rate will berequired. The illustrated graph is only an example and a similar effectcan be noticed for other bit rate ranges.

[0062]FIG. 1c illustrates another schematic graph which shows asimplified relation between the OSNR (being a direct function of powerP) and the BER for different optical bit rates. In the drawing, thecurves marked 1, 2, 3 respectively reflect channels carrying opticalsignals with different bit rates, where Bit rate (1)>bit rate (2)>bitrate (3).

[0063]FIG. 2 shows how a multi-channel optical system 10, such as a WDMsystem, can be regulated based on the proposed method. Each of theoptical channels 12, 14, . . . N of the system comprises a transmittingterminal marked with the same numbers 12, 14, . . . N, a receivingterminal and optical equipment there-between. Data transmitted via theoptical channels, using different wavelengths λ1 to λN and different bitrates, is multiplexed by a WDM multiplexer or combiner 16 andtransmitted via an optical fiber 18 in the multiplexed or combined form.The data transmitted in this form via the optical fiber 18, passesvarious active and passive units which will be mentioned below, istransmitted via a farther optical fiber 19, and is finally demultiplexedby a WDM demultiplexer (decomposer) 20 into optical channels 12′, 14′ .. . N′. It should be emphasized that the proposed method is alsoapplicable to channels, which are added or dropped on the way, and notonly to those which remain the same from the transmitting to thereceiving point; specific steps and relevant features will be discussedfurther below.

[0064] All signals received in the respective optical channels 12′, 14′,. . . N′ at the receiving terminal point are characterized by their BERvalues and other parameters (OSNR, CD, etc.) which can be directly orindirectly measured or calculated. Making decision on pre-emphasis ineach of the optical channels depends not only on the plurality of theparameters which can be obtained at the terminal point, but also onso-called “given network parameters”, for example—priorities which arestated for the system channels. A control unit CU1 22 having a memoryblock and a processor, is responsible for collecting the necessaryinformation from each of the channels (such as values of OSNR, CD, and,of course, the Bit Error Rate or Q); it also stores the networkparameters such as FEC—Forward Error Correction feature and the priorityratios, if implemented in the system. The Control Unit (CU1) 22accomplishes quite a complex multi-parameter and multi-purposeprocessing of the obtained and stored parameters and issues pre-emphasisdecisions for respective channels of the system in order to bring to theminimum the relative and absolute BER values of the optical channels atthe termination point.

[0065] The Control Unit 22 is capable of instructing the transmittingterminals of the channels to adjust their bit rates (solid arrows 24),thereby performing pre-emphasize of the optical channels. Optionally,CU1 22 is capable of affecting the channels from the point of suchparameters as:

[0066] 1) Power (dashed lines 26) which may be initiated by the controlunit 22 as a result of the processing. The power may be adjusted at thetransmitting terminals 12, 14 . . . N (thus performing a so-called powerpre-emphasis), and/or at one or more regulating or equalization units(only two units 27, 28 are shown). Each of such units is responsible ofaffecting various optical equipment at an intermediate point of thetransmission line and in different optical channels. For example, theregulating unit 27 or 28 may comprise a demultiplexer (DEMUX) splittingthe combined signal transmitted via the fiber 18 into the initialoptical channels, and a multiplexer (MUX) recombining them; each opticalchannel between the MUX and DEMUX can be provided with equipment piecesfor variable optical attenuation or amplification, for CD compensation,for PMD compensation, etc. The power emphasis is usually provided basedon OSNR value measured at the receiver terminal of each of the channelsand by applying higher power to channels having lower OSNR values andvice versa, while taking into account the recommended priority ratio(s).The power emphasis can be thus provided via the regulating units 27, 28(arrows 26). Optical amplifiers 32 and 37 symbolize so-called lineamplifiers or boosters which uniformly affect all optical channels andsimultaneously bring noise which alters the OSNTR value in each of thechannels.

[0067] 2) Chromatic Dispersion, Polarization Mode, Non-Linear effects,and others. This capability is generally illustrated by dash-pointarrows 30 entering the regulating units 27 and 28 and symbolizingaffecting the relevant equipment units in the transmission line.

[0068] The system 10 illustrated in the drawing comprises am exemplaryoptical channel “X” formed on the wavelength λx, which is dropped andreplaced (added) on the way. To this end, the system 10 comprises anadd/drop multiplexer 31, by means of which the optical signal usingwavelength λx outgoes the transmission line and is forwarded to: aclient (not shown) via a control unit CU2 marked 35. To utilize thevacant optical channel, it is captioned by another optical signal whichis added at the multiplexer 31 by a transmission terminal 33 and isindicated as λx′. The added optical signal λx′ is received at thecorresponding receiving terminal after the demultiplexer 20, parametersof this optical signal are entered to the control unit (CU1) 22 with isparameters of all other optical channels 12′, 14′ . . . N′, and thepre-emphasis decision with respect to this channel is forwarded to thecorresponding elements of the channel: its transmitting terminal 33(arrows 24 and 26) and, optionally, to the regulating unit 28 (arrows 30and 26). With respect to added signals like λx′, the control unit 22 mayissue instructions to perform power de-emphasis (power attenuation) ofsignals passing shorter distances.

[0069] It should be noted that the control block (CU2) 35, serving asingle dropped optical channel λx, operates according to the proposedmethod with respect to this particular optical channel. The blockcollects information on parameters of the optical signal (comprisingQ/BER, OSNR, CD and the like) and issues pre-emphasize instructions asto the bit rate (arrow 24) power (arrow 26) and other system parameters(arrow 30). The instructions are applied to the transmission terminal“X” of the channel and, optionally, to the relevant regulating unit 27of the channel before the add-drop multiplexer 31.

[0070] As can be understood from the above, existence of added anddropped optical channels in the transmission line, as well as existenceof regulating units in the line make it possible to perform bit rate,power and other parameters regulation at intermediate points within thecommon transmission line for achieving the required BER. In particular,the regulating units allow the emphasis or de-emphasis of power, andequalization of CD, NL and other parameters at arbitrary points in theoptical path.

[0071] While the invention has been described with reference toparticular examples, it should be appreciated that other examples andvarious implementations can be provided to illustrate the principle ofthe invention and are to be considered part thereof.

1. A method of performing pre-emphasis of one or more optical channelsrespectively carrying one or more optical signals, the method comprisesreducing bit rate of said one or more of the signals, thereby adjustingBER value thereof.
 2. The method according to claim 1, comprising stepsof a) obtaining BER value for the one or more said optical channels, b)reducing bit rate in one or more of the optical channels having theobtained BER value exceeding a maximally accepted absolute or relativeBER value thereby adjusting the BER value thereof.
 3. The methodaccording to claim 2, Per comprising a preliminary step of defining themaximally accepted absolute BER value for said one or more opticalchannels.
 4. The method according to claim 2, for a multi-channeloptical is system, the method comprises: determining Quality of Servicepriorities for said optical channels as a set of priority ratios,defining required ratios of BER values of the optical channels as alogically inverse set of said set of priority ratios, and regulating theobtained BER values of the optical channels under constant of saidpriorities by adjusting bit rates of at least one of the opticalchannels to obtain real ratios of BER values of the optical channelssubstantially equal to said required ratios.
 5. The method according toclaim 4, wherein the step of determining the set of priority ratios isbased on prices stated for transmission over different optical channels.6. The method according to claim 4, wherein the step of determining theset of priority ratios is based on at least one required Quality ofService (QoS) parameter.
 7. The method according to any one of claims 4to 6, wherein said steps are performed as follows: the determining ofthe set of priority ratios being performed by finding relations ofstated priorities for a group of the optical channels, the defining ofthe set of required BER values if performed by considering said set ofpriority ratios as logically presenting an inverse set of ratios betweenthe required BERs of said group of the channels, the regulation beingprovided by obtaining current BER values at each optical channel of saidgroup and forming a set of real BER ratios for the same group of theoptical channels, comparing the formed set of real BER ratios and saidset of A d BER ratios, and if not substantially equal for at least oneof said channels, performing the adjustment in said at least one opticalchannel, obtaining BER values for at least the charnel(s) where theadjustment were performed and forming a set of new BER ratios for saidgroup, comparing the set of required BER ratios with the set of new BERratios, continuing the adjustment until the set of new BER ratios issubstantially equal to the set of required BER ratios.
 8. The methodaccording to claim 7, wherein the step of determining the set ofpriority ratios comprises constructing said ratios based on a priorityof one of said optical channels selected as a reference to the remainingones from the group.
 9. The method according to any one of claims 2 to8, wherein the step of obtaining the BER values comprises determiningthereof based on the function BER=½*erfc(Q/{square root}2), where Q isQuality factor of the signal, measured directly.
 10. The methodaccording to any one of claims 2 to 8, wherein the step of obtaining theBER values comprises determining thereof based on the function presentedin equation (2).
 11. The method according to any one of the precedingclaims, for regulating BER value in one or more optical channels, eachcomprising a transmitter terminal, a receiver terminal, one or moreamplifiers and dispersive fiber elements there-between, the methodfurther comprising a step of additional adjusting, in said at least onechannel, one or more physical parameters from a non-exhaustive listcomprising power (P), and characteristics of dispersion.
 12. The methodaccording to claim 11, wherein the step of additional adjusting isperformed at an arbitrary point between a transmitting terminal and acorresponding receive terminal.
 13. The method according to claim 11 or12, wherein the step of addition adjusting comprises powerde-emphasizing with respect of the optical signals passing shorterdistances.
 14. A system for performing pre-emphasis in at least oneoptical channel for transmitting data at a particular bit rate, thesystem being capable of obtaining BER value at a terminal point of saidat least one optical charnel, processing said obtained BER value andreducing the bit rate at a transmission point of said at least oneoptical channel for regulating the BER value.
 15. A system forperforming pre-emphasis in one or more optical channels carrying opticalsignals at respective bit rates, capable of performing the methodaccording to any one of claims 1 to 13.